Unfortunately, this is not possible. What concerns international ticketing, the Swiss Travel Pass does not offer any benefits. GA option is for GA subscribers only.
Though, to partially use your STP, you have to split the tickets:
- a ticket Milan to Chiasso or Domodossola, including a seat reservation
- from Chiasso or Domodossola, you can use the Swiss Travel Pass, with optional seat reservation; no through seat reservation possible